How to return a promise with async/await that doesn't end in undefined?

October 30, 2022

I’m doing an async/await addition function, using a given promise-creating function which should take 100ms before being called.

const delayedPromise = ms => new Promise(resolve => setTimeout(resolve, ms));

The first thing I don’t understand is why there is no reject in the above Promise, and how do I handle the reject part if an error is thrown in my add function.

My code so far is

const delayedPromise = ms => new Promise(resolve => setTimeout(resolve, ms));

function validation(num1, num2) {
  if (!Number.isFinite(num1) || !Number.isFinite(num2)) {
    throw new Error('Only numbers are allowed');
  }


function add(num1, num2) {
  validation(num1, num2);
  return num1 + num2;
}

// This is where I need some directions 
const addAwait = async (num1, num2) => {
  const result = await delayedPromise(add(num1, num2), 100);
  return result;
};

// My test is the following
describe.only('await calculator', () => {
  it('add works fine', async () => {
    const result = await calculator.addAwait(1, 2);
    assert.equal(3, result);
  });

I’m not passing my test and I don’t understand why, it gives me AssertionError [ERR_ASSERTION]: 3 == undefined. I’m passing the two numbers to my async function, then I use the delayedPromise to create the promise and setTimeout to 100ms. It should then return a promise with the result (or error) and should make my test pass, but it doesn’t. Can someone please give me some tips as to what I’m doing wrong? Thank you

>Solution :

first, delayedPromise is a bit misleading. This is just a delay or pause or wait. It’s not supposed to return anything but a Promise that eventually resolves.

const pause = ms => new Promise(resolve => setTimeout(resolve, ms));

// ...

const addAwait = async (num1, num2) => {
  await pause(100);  // wait for 100ms
  // then do the calculation and return the result.
  return add(num1, num2);
};